It's been stated Metroid Prime 3 is the last of the Prime series, so I'd imagine they'd just create a new subseries under a different name. I mean, according to Wikipedia, Metroid 2 is actually the chronological sequel to Metroid Prime 3, so I'm sure they'd find some sort of way to continue the series.
Source (and yes, I know it's Wikipedia, and should be taken with a grain of salt, but I know little about the Metroid games anyway so really this was the only way to reaffirm my belief that MP3 is the last of its kind)
Anyway, I'm tearing through Brawl, and having tons of fun. Best in the series yet, imo.